The present invention relates to the field of determination of breathing disorders and more particularly to the determination of Apnea and Hypopnea and in particular of the type of Apnea.
Apnea is defined as cessation of breathing activity for a certain duration (6-20 seconds). Hypopnea is either slow breathing or shallow breathing or both. Detection of apnea/hypopnea (AH) is important in the diagnosis of the sleep apnea syndrome (SAS), a condition that afflicts 2% of adult females and 4% of adult male population in western countries. Detection of AH is also important in the diagnosis of breathing irregularity conditions in children and elderly patients. The diagnosis of SAS is traditionally performed in a sleep lab, but in recent years devices for home analysis of performance have been developed to offset the substantial cost and inconvenience of a full sleep lab study.
Detection of AH in a sleeping subject requires identification of actual air flow into and out of the thorax. Thus, mere monitoring of chest motion as performed with impedance pneumography is not sufficient. The existing technology is based on the use of thermal sensors in the nostrils and/or the use of chest and abdomen volume monitors (for example, inductive plethysmography which is available under the trade name Respitrace(trademark). These methods either require instrumentation of the patient""s face or use of circumferential belts around the chest and abdomen. Both approaches impose substantial inconvenience and are prone to data corruption.
One aspect of apnea detection is the prevention of sudden infant death syndrome (SIDS), the leading cause of death in infants 1-12 months old in western societies. While the etiology of SIDS is obscure, it is broadly believed that if the cessation of breathing is detected in time and resuscitate measures are taken the baby can be saved. Based on this notion many monitoring devices for home use have been introduced into the market. Unfortunately, the technology of many such devices results in many false alarms being activated, often wearing the mental stability of the parents to the point where use of the device is discontinued.
Most of the existing devices rely on monitoring the electrical impedance of the chest However, the chest impedance shows continued substantial oscillations when the breath is held due to the action of the heart within the chest. To prevent this cardiac activity from being detected as breathing activity, the detection algorithm parameters and threshold must be set such that false detection of apnea is almost inevitable. In practice, chest impedance measurements appear to be incapable of providing a clear indication of apnea without at the same time causing an undesirably high number of false alarms.
In PCT application PCT/IL97/00318, filed Sep. 30, 1997 and published as WO 98/144116 on Apr. 9, 1998, the disclosure of which is incorporated herein by reference, the present inventor described a breath sounds analysis system which includes a methodology for the detection of breath sounds in which at least one breath related sensor is placed around the respiratory system of a patient for measuring breath sound data signals; a breath analyzer which continuously matches the breath sound data signals produced by the at least one breath related sensor to at least a regular breath sound template to determine the presence of breathing and which provides an alert indication when no breathing is present for a time period longer than a given period.
In addition, the PCT application describes a method of determining the state of breathing of a patient, the method comprising determining the inspiration/expiration phase of a breath from chest movement data and defining a breath phase variable therefrom; if the tracheal breath sound data are significant and if the external noise is low determining if the tracheal breath sound data has a generally normal shape; and if not determining if the lack of flow indicates the presence of apnea and, if so, setting an apnea alarm. Preferably, the method includes generating a loud noise indication if the breath shape is not normal.
In a preferred embodiment of the invention described in the PCT application, the amplitude of the level of breath sounds are compared with an adaptive breath sounds threshold during consecutive periods of time. If the sound level is below a given level or if the detected sound does not match the spectral characteristic of authentic tracheal breath sounds, for a number of consecutive time periods an alarm may be sounded.
In preferred embodiments of the invention described in the PCT application, the breath sounds are identified as authentic breath sounds by matching the spectrum of the sounds to a spectrum which is determined during a reference period and/or by determining whether the spectral shape of the sounds, as characterized by certain parameters, is normal.
One aspect of the present invention presents an improved method and apparatus for the determination of apnea and hypopnea.
In a preferred embodiment of a method of the present invention, spectral power is used to determine the presence or absence of breath sounds. In a preferred embodiment of the invention, the average or integral of the sound power amplitude over a particular frequency range is used as a measure of the total breath flow. If the measure is below a threshold, an hypopnea alarm is preferably activated. If the measure is below a second, lower value, an apnea alarm is preferably activated.
In accordance with a further aspect of the invention, a running time average of the spectral power is used in the determination of apnea and/or hypopnea. The averaging time is made relatively short when it is desired to test for apnea in SIDS and relatively longer when sleep testing of adults is performed.
A third aspect of the invention is related to the measurement of breath sounds in the presence of noise. In the above referenced PCT application, an ambient sound sensor was used in order to determine the level of ambient sound. If the level of sound was too high, then no measurements were taken. In critical testing, such as SIDS testing, it is important to continue the testing even in the presence of some noise, as might be present in a household, such as a television in an adjoining room or the like.
In a preferred embodiment of the invention, the ambient sound is treated in the same manner as the breath sound. For frequencies at which the ambient sound level is higher than a certain level, the breath sound spectrum at that frequency is ignored in determining the breath sound power and flow. If the power of the ambient sound is above the certain level for too great a portion of the relevant spectrum, the measurement of breath power is aborted and an ambient noise indication is activated.
A fourth aspect of the invention involves the differentiation between Obstructive Sleep Apnea (OSA) and Central Sleep Apnea (CSA). While both these conditions involve cessation of breathing, with potentially lethal implications, the treatment for the two conditions is different. In particular, OSA is generally treated surgically or by a pressure device that overcomes the obstruction and CSA is not helped by either of these therapies and generally requires a neurological evaluation to determine treatment.
In accordance with a preferred embodiment of the invention, the two conditions are differentiated by determining if there is chest motion (expansion and contraction). When there is expansion and contraction of the chest, but no tracheal breath sounds, this absence of sound is presumed to be caused by an obstruction and the Apnea is classified as of the OSA type. If there are neither breathing movements nor sound, then the apnea is presumed to be centrally caused and the apnea is presumed to be of the CSA type.
There is thus provided, in accordance with a preferred embodiment of the invention a method of differentiating between OSA and CSA comprising:
determining if chest motion above a chest motion threshold is present;
determining if tracheal breath sound above a breath sounds threshold are present;
classifying a state as OSA if the chest motion is above the threshold and the sounds are below the threshold; and
classifying a state as CSA if both the chest motion and sounds are below their respective thresholds.
Preferably, determining is performed on time segments of the chest motion and breath sounds.
Preferably, the chest motion threshold is determined based on a percentage of motion during normal breathing. Preferably, the chest motion threshold is between about 5% and about 10%. Preferably, the chest motion threshold is about 10% of a normal breathing chest motion.
In a preferred embodiment of the invention, the normal breathing is normal breathing during sleep.
In a preferred embodiment of the invention, determining whether tracheal breath sounds are present includes:
producing a spectrum of the breath sound signal;
summing averaging the spectrum over a given frequency range to produce a breath sounds power signal.
Preferably, the method includes producing a separate breath sound spectrum for each of a plurality of given time periods.
In a preferred embodiment of the invention, the method includes:
producing a breath sounds power signal representative of the breath sounds power in a plurality of given time periods; and
integration circuitry which receives the breath sounds power signal; and
producing a running time average or integral of the breath sounds power signal over a second given time period.
There is further provided, in accordance with a preferred embodiment of the invention, breath sounds apparatus for classifying a breathing state comprising:
a chest motion sensor and produces a chest motion signal;
a tracheal breath sound sensor that produces a breath sound signal;
computing circuitry that receives the breath sound signal and the chest motion signal and determines if chest motion above a chest motion threshold is present and if tracheal breath sound above a breath sounds threshold is present and classifies a breathing state as OSA if the chest motion is above the threshold and the sounds are below the threshold and classifies a breathing state as CSA if both the chest motion and sounds are below their respective thresholds.
In a preferred embodiment of the invention, the computing circuitry includes:
spectrum producing circuitry which receives the breath sound signal and produces a spectrum of the breath sound signal; and
averaging circuitry which receives the spectrum and sums or averages the spectrum over a given frequency range to produce a breath sounds power signal, wherein the breath sounds power signal is used in the determination of whether the breath sounds are above their threshold.
There is further provided, in accordance with a preferred embodiment of the invention, apparatus for the detection of breathing activity, comprising:
a breath sound sensor which produces a breath sound signal responsive to breath sounds of a subject;
spectrum producing circuitry which receives the breath sound signal and produces a spectrum of the breath sound signal;
averaging circuitry which receives the spectrum and sums or averages the spectrum over a given frequency range to produce a breath sounds power signal.
In preferred embodiments of the invention the given frequency range has a lower frequency limit of at least 200 Hz, 250 Hz, 300 Hz or 400 Hz. In preferred embodiments of the invention the given frequency range has an upper frequency limit of 1200 Hz or less, 1300 Hz or less, 1400 Hz or less, 1500 Hz or less, 1800 Hz or less or 2000 Hz or less.
In a preferred embodiment of the invention the spectrum producing circuitry produces a separate breath sound spectrum for each of a plurality of given time periods.
There is further provided, in accordance with a preferred embodiment of the invention, apparatus for the classification of breathing activity, comprising:
a breath sensor which produces a breath sound signal responsive to breath sounds of a subject;
breath sounds power circuitry which receives the breath sounds signal and produces a breath sounds power signal representative of the breath sounds power in a plurality of given time periods; and
integration circuitry which receives the breath sounds power signal and produces a running time average or integral of the breath sounds power signal over a second given time period.
Preferably, the breath sounds power circuitry comprises:
spectrum producing circuitry which receives the breath sound signal and produces a spectrum of the breath sound signal; and
averaging circuitry which receives the spectrum and sums or averages the spectrum over a given frequency range to produce the breath sounds power signal.
In preferred embodiments of the invention the given time period is greater than 20, 50 or 75 msec. In preferred embodiments of the invention, the given time period is shorter than 200, 150, 100, 75 or 50 msec.
In accordance with a preferred embodiment of the invention, the apparatus includes integration circuitry which receives the breath sounds power signal and produces a running time average or integral of the breath sounds power signal over a second given time period.
In preferred embodiments of the invention the second given time period is at least 5 seconds, about 6 seconds, about 8 seconds, about 10 seconds, between about 10-15 seconds or between about 15-20 seconds.
Preferably, the apparatus includes comparison circuitry which receives the averaged or integrated breath sounds power signal and produces an apnea indication if the averaged breath sounds power signal is below a given apnea threshold. Preferably the given apnea threshold is based on a breath sounds power signal acquired during normal breathing. Preferably, the given apnea threshold is about 10% of the a breath sounds power signal acquired during normal breathing. Preferably, the given apnea threshold is between 5% and 10% of the a breath sounds power signal acquired during normal breathing.
Preferably the apparatus includes comparison circuitry which receives the averaged or integrated breath sounds power signal and produces an hypopnea indication if the averaged breath sounds power signal is below a given hypopnea threshold. Preferably, the given hypopnea threshold is based on a breath sounds power signal acquired during normal breathing. Preferably the given hypopnea threshold is about 25% of the a breath sounds power signal acquired during normal breathing. Preferably, the given apnea threshold is between 20% and 30% of the a breath sounds power signal acquired during, normal breathing.
In a preferred embodiment of the invention, the apparatus includes:
an ambient sound sensor which produces an ambient sound signal responsive to ambient sounds;
spectrum producing circuitry which receives the ambient sound signal and produces a spectrum of the ambient sound signal;
comparison circuitry which compares the spectrum of the ambient sound spectrum with a threshold spectrum and produces a signal for those frequencies for which the spectrum is greater than the threshold; and
spectrum conditioning circuitry which conditions the breath sound spectrum by replacing the value of the breaths sounds spectrum by a different value for those frequencies for which the ambient sound spectrum exceeds the threshold.
There is further provided, in accordance with a preferred embodiment of the invention, apparatus for conditioning a breath sound signal to reduce the effects of ambient sound, comprising:
a breath sound sensor which produces a breath sound signal responsive to breath sounds of a subject;
spectrum producing circuitry which receives the breath sound signal and produces a spectrum of the breath sound signal;
an ambient sound sensor which produces an ambient sound signal responsive to ambient sounds;
spectrum producing circuitry which receives the ambient sound signal and produces a spectrum of the ambient sound signal;
comparison circuitry which compares the spectrum of the ambient sound spectrum with a threshold spectrum and produces a signal for those frequencies for which the spectrum is greater than the threshold; and
spectrum conditioning circuitry which conditions the breath sound spectrum by replacing the value of the breaths sounds spectrum by a different value for those frequencies for which the ambient sound spectrum exceeds the threshold.
Preferably the threshold is based on an ambient sounds spectrum produced in the absence of substantial ambient sound. Preferably, the threshold at a given frequency is determined, from a plurality of spectra of the ambient sound, as the average of value of the spectrum at the given frequency plus a factor times the standard deviation of the values of the spectrum. Preferably, the factor is more than 3 or between 4 and 6.
In a preferred embodiment of the invention the spectrum conditioning circuit replaces the value of the breaths sounds spectrum by a zero for those frequencies for which the ambient sound spectrum exceeds the threshold. Alternatively, the spectrum conditioning circuit replaces the value of the breaths sounds spectrum by a value equal to the average of values for adjacent frequencies, for those frequencies for which the ambient sound spectrum exceeds the threshold.
There is further provided, in accordance with a preferred embodiment of the invention, a method differentiating between OSA and CSA comprising:
determining if chest motion above a chest motion threshold is present;
determining if tracheal breath sound above a breath sounds threshold are present;
classifying a state as OSA if the chest motion is above the threshold and the sounds are below the threshold; and
classifying a state as CSA if both the chest motion and sounds are below their respective thresholds.